1. Field of the Invention
This invention generally relates to wind-driven power generators. More specifically, this invention relates to omnidirectional vertical-axis wind turbines adapted for driving a variable speed, constant frequency electric power generators and other utilization devices.
2. Description of Related Art
There is increasing interest in developing apparatus for converting wind energy into electric power. Diverse implementations of various concepts have included conventional windmills. However, conventional windmills are unidirectional. That is, they are most efficient only when the axis of rotation of the windmill aligns with the wind direction. Therefore to be efficient a windmill must include some means for aligning the windmill's blades by rotating the axis of rotation about a vertical axis.
Aligning such a rotating mass requires significant power. As wind direction constantly fluctuates, correction must be applied essentially continuously. The power required to rotate or position the rotating windmills decreases the overall efficiency of the system. In some situations where the wind direction fluctuates rapidly, such aligning means may be unable to react in a timely fashion. Therefore, the efficiency is further reduced.
These issues have led others to develop turbines that operate about horizontal or vertical axes. Each has advantages and disadvantages. The primary disadvantage of horizontal axis wind turbine is that it is not omnidirectional. Some alignment means must be still be provided to align the wind turbine about a vertical axis.
To overcome these disadvantages of conventional windmills and horizontal axis turbines, effort has been placed into the development of wind turbines with wind-driven blades that rotate about vertical axes and that do not require a structure for rotating the entire mechanism for aligning the wind turbine with the wind direction. For example, For U.S. Pat. No. 4,508,973 to Payne discloses a wind-operated electrical generator system with a stationary circular arrangement of segmental wind inlet passages. Each passage has vertical inlet openings and inwardly and upwardly curving walls. The lower and upper walls form an inwardly and upwardly extending convergence to form constricted upwardly directed exit passage that merges into a Venturi throat. A bladed impeller mounts in the throat and responds to the wind in the throat that is characterized by increased wind speed to rotate a that impeller in the throat.
U.S. Pat. No. 5,391,926 to Staley et al. discloses a wind turbine adapted for high-wind conditions. Fixed stators direct wind currents having horizontal components into a rotor assembly. Rotors in this assembly respond to the wind currents by imparting rotation upon a central vertical shaft. The stators are designed with a double-curve which perform at levels higher than either straight or single-curve stators. Thus the structure is stated to be adapted for operation with high-energy potential winds.
U.S. Pat. No. 7,189,050 to Taylor et al. discloses a cross-flow wind turbine in which an air foil stator accelerates wind along its surface and creates a low pressure area on the leading face of a rotor blade during a power stroke. A blocking stator prevents wind from impeding the movement of the rotor blades during the return cycle and drives wind under the trailing face of the rotor blades during the power cycle. This creates a large pressure differential between the leading and trailing faces of the rotor blade during the power cycle. In some embodiments gaps are provided between the inside edge of the rotor blade and the stationary shaft to vent wind collected by the rotor blade during certain portions of the rotation cycle. This vented wind increases the pressure on the trailing face of the rotor blades during a return cycle.
U.S. Pat. No. 7,329,965 to Roberts discloses an aerodynamic-hybrid, vertical-axis wind turbine that includes a rotor air foil and stator blade combination. This structure increases wind velocity and pressure while eliminating back pressure and improving the laminar flow of wind around and through the device. Each rotor air foil has a horizontal cross section with a crescent shape including a convex leading side and a concave trailing side with a thicker middle section. Each stator blade has a horizontal cross-section with a planar side and a convex side. Each rotor air foil and stator blade combination is secured between upper and lower annular sails.
U.S. Pat. No. 7,400,057 to Sureshan discloses an omni-directional wind turbine that includes a throat section that has an air foil, multi-bladed rotor. The intake of a shroud incorporates multiple horizontally curved blades of torroidal form varying up to nearly twice the size of the rotor diameter. These are stacked, staggered and secured in place by multiple aerodynamic vertical walls in the radial direction. Wind is collected in a central collection chamber. Blade curving angles and stacking arrangement are established to accelerate and focus the wind across the full swept area of the rotor blades.
U.S. Patent Pub. 2003/0156938 to Verini discloses apparatus for capturing and harnessing wind energy. A tower has the form of an elongated vertical conduit. A wind collector at a first end portion of the conduit deflects the wind into the conduit to create an axial flow toward the second end that carries a wind turbine mounted for rotation about a vertical axis.
Each of these designs has specific deficiencies. A number, including conventional windmills, are inefficient in converting the wind's energy into energy that rotates a shaft connected to an electric power generator or other utilization device. Each is a single-stage structure that can not convert all the energy, so considerable unconverted energy remains in the exhaust.
What is needed an omnidirectional vertical axis turbine that extracts the maximum energy from the wind for driving a mechanical or electrical utilization device, such as a variable speed, constant frequency, power generator.